I think the filaments are in series in the 922, so you cannot pull one
tube and test the amp that way.
73, Rob, NC0B
Tom Cathey wrote:
Hi Clay,
That's good - you've made some progress and eliminated 1/2 of the rig.
I take it you have no good 3-500Z spares to try. If not, I wouldn't worry
about blowing anything up by popping fuses. Pull one tube out and fire it
up. If the amp works, try to load it up on 2100V to see if the amp puts out
some RF. If it does, then switch to the other single tube and see if it pops
the fuse. It's a 50-50 guess if it's only one tube.
It could be other things too, but this is the shortest path to get right to
the heart of the problem.
BTW, do the fil pins have ample air going by them? Your overheated fil choke
might be a clue that there's no air flow below the chassis. I have two
3-500Z amplifiers here. One has chimneys and is FB. The other had just a fan
and no air below. It was melting solder out of the fil pins. I ended up
adding a blower below to correct this and all is FB. ie, Hopefully you
didn't damage a tube's seals and loose the vacuum, etc. I assume that they
both still light up OK?
Check the tubes as above and post again.
73,
Tom, K1JJ
Thanks for the responses. I didn't think about an internal tube short as
a
possibility. I removed the plate caps from the 3-500's and the HV
measures
normal (2100V CW, 3100V SSB). Is it logical to assume that one (or
possibly
both) of the 3-500's has developed an internal problem or should I be
looking for other causes as well, such as a bias supply problem. I don't
have access to a hy-pot tester, but a simple resistance check from
filament
to grid on both tubes doesn't reveal a short.
One thing I didn't mention earlier (because I don't believe it's relevant)
is that the small filament choke (L20) that connects to the filament
transformer center tap, has overheated in the past. Since one of the tube
filaments came unsoldered from the pin (a simple soldering job fixed it),
I
figure all of the extra current flowing through the small choke was the
cause of that. Other than some discoloration, it appears ok.
Any and all suggestions are welcome. I don't want to risk my power supply
by blowing more fuses trying to determine which of the tubes might be at
fault.
73,
Clay W7CE
----- Original Message -----
From: "K7RDX" <k7rdx@earthlink.net>
To: "Clay Curtiss W7CE" <w7ce@curtiss.net>
Sent: Saturday, December 04, 2004 5:42 PM
Subject: Re: [Amps] TL-922A - Loud Pop, Blown Fuses
Hi Clay, pull the tubes and start the amp again. If plate voltage is
normal
and the fuse doesn`t blow again, shut everything down and inspect the
tube
sockets for burnt or damaged rf chokes from the three grid pins to
ground.
If one of the tubes flashed over, some of these chokes will probably be
destroyed or at least damaged. 3-500z tubes sometimes develop a short
from
grid to filament after a period of use and will flash over on the next
start-up. If you have access to a hypot tester, a shorted tube is easy
to
spot...Hope this helps you find the problem...A flashover might damage
the
bias as well as the high voltage supply. 73,Jim.
----- Original Message -----
From: "Clay Curtiss W7CE" <w7ce@curtiss.net>
To: <amps@contesting.com>
Sent: Saturday, December 04, 2004 12:05 PM
Subject: [Amps] TL-922A - Loud Pop, Blown Fuses
Yesterday morning when I turned on my TL-922A, there was a very loud
pop
followed by blown fuses. I replaced the fuses, changed the setting to
CW
to
reduce the B+ voltage and tried again. Similar result with several
smaller
pops and then blown fuses. I use a footswitch for T/R control and it
wasn't
depressed nor was I transmitting with the exciter when I powered up.
I
should also mention that I've heard a pop on power up two times
previously,
but neither time caused a blown fuse. I opened it up today for a
visual
inspection and see no signs of arcing or anything unusual.
Before I dig into this thing, has anyone else experienced this
problem?
Any
hints or suggestions to reduce the troubleshooting time will be
appreciated.
Thanks and 73,
Clay W7CE
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